In general, a solid oxide fuel cell (SOFC) comprises a pair of electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of about 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:
Anode reaction:H2+O═→H2O+2e−CO+O═→CO2+2e−CH4+4O═→2H2O+CO2+8e−
Cathode reaction:O2+4e−→2O═
Known SOFC designs include planar and tubular fuel cells. Applicant's own PCT application no. PCT/CA01/00634 discloses a method of producing a tubular fuel cell by electrophoretic deposition (EPD). The fuel cell comprises multiple concentric layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube.
It is also known to arrange a plurality of tubular fuel cells in an array or “stack” to increase electrical output. Designs have been proposed for stacking together relatively large-diameter (≧5 mm) thick-walled tubular fuel cells that are essentially self-supporting; for example it is known to stack large diameter tubular fuel cells having diameters in the order of about 20 mm in a grid-like pattern and interconnect the fuel cells with nickel felt spacers. This and other known designs for large diameter self-supporting tubular fuel cells are not particularly well suited for small diameter fuel cells (≦5 mm), especially if such small diameter fuel cells are arranged into a tightly-packed array. It is therefore desirable to provide an improved stack design that enables the close-packing of a plurality of small-diameter tubular fuel cells, and a system for such stack.